1. Field of the Invention
The present invention relates to the measurement of heat flux through a portion of the surface of a heat exchanger tube, by use of a heat flux meter mounted on the surface of the inside wall of the heat exchanger tube. The heat flux meter is capable of measuring the high heat fluxes encountered in a boiling heat transfer system.
2. Description of the Prior Art
Many heat flux meters or heat flow measurement devices have been disclosed in the prior art that are capable of measuring heat flux (or heat flow) into or out of heat exchanger tubes. These heat flux meters are designed to possess a low thermal resistance, preferably lower than the thermal resistance of the heat exchanger tube itself in order not to intrusively alter the measured value of the heat flux passing through the heat exchange tube. The thermal resistance of these heat flux meters must also be known in order to correct for their presence in the thermal circuit consisting in part of the meter and the tube.
A typical heat flux meter is disclosed for example in U.S. Pat. No. 1,528,383. The device in this '383 patent uses a plurality of thermopiles arranged on a measuring plate, the thermopiles or thermocouples consisting of alternate wires of copper and constantan coupled or soldered together. The thermocouples are typically arranged in an array of hot junctions separated by a relatively thick layer of insulation from a corresponding array of cold junctions, well known to the art. Due to the heat flow through the measurement device a higher temperature exists at the hot junctions than at the cold junctions. A resultant electric current is generated due to this temperature difference which when measured indicates the amount of heat flux passing through the surface area monitored by the thermocouple apparatus. It should be noted that the insulation used between the array of hot and cold junctions increase the overall thermal resistance of the device.
In considering the use of the '383 device in high heat flux applications it must be remembered that extremely high heat fluxes in the order of 100,000 BTU/hr. FT.sup.2 are typically encountered in boiling heat transfer system designs. As can be imagined, the soldered thermocouple system of the '383 device with its relatively high thermal resistance and questionable reliability at these high heat flux levels cannot be used to measure the heat fluxes encountered in a boiling heat transfer system.
If a device of this design were mounted within a heat exchanger tube that had fluid flowing through it, the thickness of the insulation layer would restrict the fluid flow through the tube.
Available materials therefore appear to be limited to thin laminated metallic sections which currently are attached to a heat transfer surface by either a clamp or ceramic cement well known to the art, for example, and are used to measure high heat fluxes, as discussed in the thermal flux meter sales literature published by International Thermal Instrument Co., P.O. Box 309, Del Mar, CA 92014.
Such thin metallic meters operate on the same principle as the thermocouple discussed earlier. Whereas in a thermocouple, however, only two dissimilar metals are connected together, in the thin metallic meters several layers of dissimilar metal may be laminated together.
It is desirable to install these thin metallic heat flux meters in a manner to minimize disruption to the normal heat flow through the surface of the heat exchange tube. In other words, the intrusive nature of the heat flux meter must be minimized. The meters therefore should not be attached to the heat exchanger tube by, for example, a process such as arc welding. The arc welding material deposited upon the heat exchanger tube would change the normal heat flow through the surface(s) of the heat exchanger tube and the voids subsequently left under the surface area of the heat flux meter after the edges of the meter are welded to the tube would disrupt the natural heat flux through the surface(s) of the heat exchanger tube, and subsequently reduce the accuracy of the signal generated by these meters. Similar problems are encountered in the use of ceramic cement.
An apparatus and method of installation of the apparatus needs to be developed therefore that minimizes the disruption of the natural heat flow through the surface(s) of the heat exchanger tube, and also minimizes the disruption of the normal fluid flow through the heat exchanger tube itself. The apparatus must also be capable of withstanding the high heat fluxes associated with a boiling heat transfer system.